UWP

[deakjahn]

It really depends on how ambitious you want to be. If a very simple solution is needed for basic texts (p, b, i, br and similar), then using the idea at http://www.visuallylocated.com/post/2015/01/16/Displaying-HTML-content-in-a-TextBlock.aspx is enough. Basically:

using System.ComponentModel;
using System.Xml.Linq;
using XXX.Controls;
using XXX.UWP.Controls;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Documents;
using Xamarin.Forms;
using Xamarin.Forms.Internals;
using Xamarin.Forms.Platform.UWP;
using Span = Windows.UI.Xaml.Documents.Span;

[assembly: ExportRenderer(typeof(HtmlLabel), typeof(HtmlLabelRenderer))]
namespace XXX.UWP.Controls {

  [Preserve(AllMembers = true)]
  public class HtmlLabelRenderer : LabelRenderer {

    protected override void OnElementChanged(ElementChangedEventArgs<Label> e) {
      base.OnElementChanged(e);

      if (Control != null && Element != null && !string.IsNullOrWhiteSpace(Element.Text))
        ConvertHtml(Element.Text ?? string.Empty, Control);
    }

    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e) {
      base.OnElementPropertyChanged(sender, e);

      if (e.PropertyName == Label.TextProperty.PropertyName && Control != null && Element != null && !string.IsNullOrWhiteSpace(Element.Text))
        ConvertHtml(Element.Text ?? string.Empty, Control);
    }

    private void ConvertHtml(string html, TextBlock control) {
      control.Inlines.Clear();
      try {
        ParseText(XElement.Parse($"<div>{html}</div>"), control.Inlines);
      }
      catch {
        Control.Text = html;
      }
    }

    private bool AddLineBreakIfNeeded(InlineCollection inlines) {
      if (inlines.Count > 0) {
        var lastInline = inlines[inlines.Count - 1];
        while ((lastInline is Span)) {
          var span = (Span)lastInline;
          if (span.Inlines.Count > 0)
            lastInline = span.Inlines[span.Inlines.Count - 1];
        }
        if (!(lastInline is LineBreak)) {
          inlines.Add(new LineBreak());
          return true;
        }
      }
      return false;
    }

    private void ParseText(XElement element, InlineCollection inlines) {
      if (element != null) {
        var currentInlines = inlines;
        string elementName = element.Name.ToString().ToLowerInvariant();
        switch (elementName) {
          case "a":
            var link = new Hyperlink();
            inlines.Add(link);
            currentInlines = link.Inlines;
            break;
          case "b":
          case "strong":
            var bold = new Bold();
            inlines.Add(bold);
            currentInlines = bold.Inlines;
            break;
          case "i":
          case "em":
            var italic = new Italic();
            inlines.Add(italic);
            currentInlines = italic.Inlines;
            break;
          case "u":
            var underline = new Underline();
            inlines.Add(underline);
            currentInlines = underline.Inlines;
            break;
          case "br":
            inlines.Add(new LineBreak());
            break;
          case "p":
            // Add two line breaks, one for the current text and the second for the gap.
            if (AddLineBreakIfNeeded(inlines))
              inlines.Add(new LineBreak());
            var paragraphSpan = new Span();
            inlines.Add(paragraphSpan);
            currentInlines = paragraphSpan.Inlines;
            break;
          case "li":
            inlines.Add(new LineBreak());
            inlines.Add(new Run { Text = " • " });
            break;
          case "ul":
          case "div":
            AddLineBreakIfNeeded(inlines);
            var divSpan = new Span();
            inlines.Add(divSpan);
            currentInlines = divSpan.Inlines;
            break;
        }

        foreach (var node in element.Nodes()) {
          if (node is XText textElement)
            currentInlines.Add(new Run { Text = textElement.Value });
          else
            ParseText(node as XElement, currentInlines);
        }

        if (elementName == "p")
          currentInlines.Add(new LineBreak());
      }
    }

  }
}

[deakjahn]

But if you want the full treatment, then the widely used MS code of https://github.com/Microsoft/WPF-Samples/blob/master/Sample%20Applications/HtmlToXamlDemo/HtmlToXamlConverter.cs can be used. The conversion is basically similar:

string xaml;
InlineCollection xamLines;
try {
  xaml = HtmlToXamlConverter.ConvertHtmlToXaml(html, false);
  xamLines = ((XamlReader.Parse(xaml) as Section).Blocks.FirstBlock as Paragraph).Inlines;
}
catch {
  return;
}

var clone = new Inline[xamLines.Count];
xamLines.CopyTo(clone, 0);

control.Inlines.Clear();
foreach (var line in clone)
  control.Inlines.Add(line);

[deakjahn]

A modification to handle HTML entities:

  public class HtmlLabelRenderer : LabelRenderer {
    private static readonly Dictionary<string, string> HtmlEntities = new Dictionary<string, string> {
      { "nbsp", "&#160;" },
      { "lt", "&#60;" },
      { "gt", "&#62;" },
      { "amp", "&#38;" },
      { "quot", "&#34;" },
      { "apos", "&#39;" },
      { "cent", "&#162;" },
      { "pound", "&#163;" },
      { "yen", "&#165;" },
      { "euro", "&#8364;" },
      { "copy", "&#169;" },
      { "reg", "&#174;" },
    };
    private static readonly string HtmlEntitiesDtd;

    static HtmlLabelRenderer() {
      string list = string.Join("", HtmlEntities.Select(ent => $"<!ENTITY {ent.Key} \"{ent.Value}\">"));
      HtmlEntitiesDtd = $"<!DOCTYPE html [{list}]>";
    }

And process the HTML like this:

      try {
        var settings = new XmlReaderSettings { DtdProcessing = DtdProcessing.Parse };
        using (var xml = new StringReader(HtmlEntitiesDtd + html))
        using (var reader = XmlReader.Create(xml, settings)) {
          var doc = XDocument.Load(reader);
          ParseText(doc.Root, control.Inlines);
        }
      }
      catch {
        Control.Text = html;
      }